A conventional automated light beam positioning system typically includes a light beam positioner and a system control computer. The light beam positioner, which typically includes optical components having movable light-directing surfaces, receives beam position and velocity command data ("command data") processed by the system control computer. The optical components have either reflective or refractive light-directing properties. The light beam positioner determines, in response to the command data, the propagation path of an incident light beam to direct it to a predetermined position on a workpiece.
To change the path the light beam travels along the workpiece, the light beam positioner changes, in response to different command data, the orientations of the movable light-directing surfaces of the optical components. One example of such a device is a galvanometer beam positioner, which employs a pair of selectively pivotable mirrors that cooperate in response to the command data to direct an incident light beam to a desired position on a target surface.
FIGS. 1A and 1B are diagrams of a prior art light beam positioning system 10 that employs a galvanometer beam positioner 12 to direct an incident light beam 14 emanating from a laser source 16 to a desired position on a planar target surface 18. FIG. 1A is a block diagram of the data processing modules for controlling galvanometer beam positioner 12, and FIG. 1B is a pictorial diagram of the optical components of light beam positioning system 10.
With reference to FIGS. 1A and 1B, a peripheral storage medium 20, such as a magnetic disk, provides the command data to a system control computer 22. System control computer 22 typically performs multiple tasks, at least some of which do not pertain to a light beam positioning operation. The command data represent "tables" of start and end positions of possible paths and their associated speeds of light beam travel along the workpiece. System control computer 22 acquires electrical measurement data and, in response to such data, selects one of each of the possible paths and speeds for the light beam. For example, command data for a laser-based resistor trimmer represents a series of respective start and end positions from which and to which laser beam 14 moves to carry out a trimming operation on a particular resistor network with a known pattern.
System control computer 22 delivers the command data to a position data generator 24, which develops an X position coordinate digital word signal ("X signal") and a Y position coordinate digital word signal ("Y signal"). The X and Y signals represent digital words that are converted to DC voltages that are delivered to different ones of two galvanometer motors (not shown). A galvanometer motor is a DC motor whose shaft is operatively connected to a mirror for movement about a pivot axis. Galvanometer beam positioner 12 employs a pair of separate mirrors 26 and 28 mounted to brackets that are operatively connected to the galvanometer motors for pivotal movement about orthogonally aligned pivot axes. Mirrors 26 and 28 receive the incident laser beam 14 and pivotally move about their respective pivot axes in response to the X and Y signals to direct beam 14 to a desired position on target surface 18. Galvanometer beam positioner 12 also includes a lens of the F-.THETA. type for focusing beam 14 and a mirror 32 for directing beam 14 toward target surface 18. The pivotal movements of mirrors 26 and 28 describe a generally circular addressable imaging area 33 on target surface 18.
One of the problems associated with light beam positioning system 10 is that the system optical components can cause unit pivotal movements of the galvanometer mirrors to not result in unit linear movements of laser beam 14 on target surface 18. A primary source of such nonlinear beam movement is the nonlinear distortion that results whenever the system optical components inaccurately convert pivotal motion to linear motion. For example, whenever it is positioned off the optic axis, galvanometer mirror 26 misdirects beam 14 as it propagates toward galvanometer mirror 28 and lens 30. The introduction of such optical distortion results in "pincushion" distortion, which represents nonequidistant spacing between adjacent positions near the periphery of target surface 18 as beam 14 moves toward them in response to corresponding unit angular displacements of galvanometer mirrors 26 and 28.
FIG. 2 is a map of an exemplary addressable field 34 showing the light beam grid pattern described on target surface 18 by unit angular displacements of galvanometer mirrors 26 and 28 in response to unit changes in command data position values. For purposes of the following description, galvanometer mirrors 26 and 28 pivotally move about the X and Y axes, respectively. FIG. 2 shows that addressable field 34 of target surface 18 is of pincushion shape with equidistant spacing of 2.5 mm between adjacent positions in the vicinity of the center 36 of the target surface, an outwardly directed contour with a "keystone" shape at the periphery portions 40 in the X direction, and an inwardly directed contour at the periphery portions 38 in the Y direction. The resulting beam positioning nonlinearity is typically a few percent near the periphery of the target surface.
There are several techniques employed in an attempt to deal with the nonlinear beam positioning introduced by the galvanometer beam positioner optical components. One technique is simply to ignore the problem and tolerate the resulting pincushion distortion. A second technique entails visual feedback in which an operator uses a joystick manually to position the laser beam at the desired start position. This technique suffers from the obvious disadvantage of being labor intensive and does not correct the problem of positioning nonlinearity that occurs as the laser beam moves from a start position near the periphery of the target surface. The use of visual feedback to direct the laser beam in the correct start position cannot, therefore, result in straight line beam positioning along the target surface.
A third technique for correcting beam positioning errors entails correcting the command data processed by system control computer 22. This is accomplished by providing in the system control computer 22 a calibration map in the form of a look-up table having correction data corresponding to the desired target surface positions to which the command data correspond. System control computer 22 uses the correction data to modify the command data to eliminate beam position errors. This error correction technique is disadvantageous because software files located in system control computer 22 must be changed each time a different beam positioner 12 is used in conjunction with system control computer 22. Another disadvantage is that the system control computer response time to a change in command data is inherently long because system control computer 22 performs many other unrelated tasks that are assigned priorities. The desired change in beam position may take place, therefore, after significant time has elapsed from the receipt of command data.